In antiquity, as now, when Roman Catholic or Eastern Orthodox Christians gather together for worship they are aware that they are not merely joining together with those Christians in the room with them, or in the city, or in the same country. Not even only with all the Christians in the world. They were joining together with every Christian who had died and gone on to heaven in worshipping God.
I know that the RC church uses the terms the Church Triumphant and the Church Militant. The Church Triumphant is the body of believers who have gone on to their final rewards before us. The Church Militant is those who are still alive on earth. Who still have to fight against the enemy. Whether that enemy would be a physical temptation, or merely an internal one.
In addition to having the surety of knowledge that the saints, the dearly departed in heaven are alive with Christ and capable of adding their prayers to ours, we know that the bodies of saints remain holy and sacred here on earth. Relics have been known to be the vehicles through which God performs miracles.
Many churches had altars built right over the graves of saints.
The altar of a Roman Catholic church, and I believe the Eastern Orthodox as well, must contain a relic of a saint. When an altar is 'decommissioned' for whatever reason, the reliquary must be removed and placed in a new altar. The icons and other art of saints that covers the inside of RC or EO churches also serve to remind the worshippers there that they are not alone. People have gone before them, and have made it to the end of the race, victorious.
"If death has truly been destroyed, then these departed friends in Christ are alive and invisibly present; they are just as genuinely present as the friends and family we can see. But now they are also in the throne room of God, and continually at prayer. The early Christians had such complete confidence that death had been overthrown, that they asked the holy departed to pray for them."